Polyethylene and polypropylene, which are representative polymers of polyolefin, are widely used in a variety of fields due to excellent physical properties and low prices. Polyethylene and polypropylene may be used in themselves in material fields requiring inherent physical properties or may be used in modified or combined forms suitable for characteristics of fields applied. Among them, polypropylene also has an extended application as a substitute for polyethylene due to excellent mechanical properties and relatively high melting point.
For example, as materials for insulation layers surrounding metal conductors of power cables, conventionally, crosslinked substances of polyolefin-based polymers such as polyethylene, ethylene/propylene elastomeric copolymers (ethylene/propylene rubbers, EPR) and ethylene/propylene/diene (EPDM) copolymers are generally used.
However, in order to recycle copper (Cu) and aluminum (Al), which are conductors of cables which occupy most of production cost, it is necessary to remove insulation layers that fail to satisfy requirements after production of cables or insulation layers of used cables. However, crosslinked polyethylene (crosslinked PE: XLPE) used as an insulation layer is removed by incineration upon disposal of used cables because reforming (remolding) of XLPE is impossible. At this time, a great amount of CO2 is generated and the burned ash should be buried in a landfill, which causes an environmental problem. In addition, so as to produce XLPE cables, a crosslinkage process at high temperature and at high pressure (for example, about 300° C., 7 atm or the like) is required after cable forming (molding). For this purpose, several tens of meters or more of curing tubes should be introduced, which requires considerable production facility site and power costs. Furthermore, addition of expensive chemical substances (crosslinking agents or the like) is necessary for crosslinkage, which disadvantageously results in increased production costs and waste of resources.
Accordingly, a variety of attempts to solve these problems have been made. For example, Korean Patent Laid-open No. 2014-0134836 suggests use, as an insulation material, of a composition in which propylene copolymer particles are dispersed in polypropylene as a matrix. In accordance with this technology, heat resistance is excellent even without crosslinkage since the polypropylene matrix has a melting point of 150° C. or higher, but a composition having a heterogeneous phase in which a propylene copolymer (rubbery substance) having a predetermined mean weight particle size is dispersed in a polypropylene matrix is used as an insulation material in order to improve insufficient flexibility, bendability and the like, which are drawbacks of polypropylene resins. Products can be manufactured immediately after forming and cooling cables, without an additional crosslinkage process, which causes considerable reduction of production costs and enables all of expensive conductors to be recycled by melting and separating insulation layers that fail to satisfy requirements after production of cables or are disposed, and then forming expensive conductors again. Consequently, it is possible to recycle 100% of the melted and separated PP insulation layer.
As another example, polypropylene is used as a base material for flame retardant substances as well. Flame retardant composites obtained by adding a rubbery polymer to improve flexibility to polypropylene and adding a great amount of inorganic flame retardant agent thereto are used.
Examples of the insulation layer material or flame retardant material have a common feature in that they have a structure in which a rubber-phase substance is dispersed in a polypropylene-based matrix. However, conventionally developed materials have several problems.
Insulation layer materials for power cables provide desired levels of flexibility and bendability through a heterogeneous-phase morphology using the incompatible principle between the polypropylene matrix and propylene copolymer particles, but have problems in that propylene copolymer particles substantially have a heterogeneous phase in the polypropylene matrix, the interfacial separation between propylene copolymer particles and the polypropylene matrix occurs upon bending of power cables or application of external mechanical stress and tension, which causes formation of micro-voids and thus a phenomenon, called “stress whitening (blush)”, in which whitening is visible by the naked eye. This results in problems of deterioration in electric and mechanical properties (see: “Comparison of Electrical Treeing in Polypropylene And Crosslinked Polyethylene”—Jorunn Hoito Erling Ildstad/Norweigian University of Science and Technology: research relating to tests showing that micro-voids cause electrical treeing, and deterioration in electric and mechanical properties).
In addition, regarding flame retardant materials, a predetermined level or more of flame retardancy is dependent upon the content of substantially added flame retardant agent. There is a technological limitation on increasing the amount of flame retardant agent added to materials containing a combination of polypropylene and a rubber-phase substance. Even though a great amount of flame retardant agent can be added, there is a problem of difficulty in maintaining general mechanical properties required for flame retardant materials. Accordingly, there is an increasing need for development of technologies that can solve these problems associated with the composition in which a rubber-phase substance is dispersed in a polypropylene-based matrix.